Scrapie is a transmissible neurodegenerative disease that is caused by infectious pathogens called prions. The only macromolecule that has been identified in the scrapie prion is an abnormal isoform of the prion protein (PrPSc), which is encoded by a host gene. The PrP gene (Prn-p in mice) also controls scrapie susceptibility and the interval between inoculation and illness. Although many studies have failed to detect a scrapie-specific nucleic acid, these negative results do not exclude the possibility that the scrapie agent has a host-independent genome. The reported existence of numerous :microbiological strains" of scrapie prions argues for a host-independent informational component. However, some properties of prion isolates, previously attributed to a nucleic acid genome, can be explained as an epigenetic effect of the PrP allotype of the previous host. Taking advantage of Prn congenic mouse strains and transgenic mice developed for these studies, experiments will determine which properties of distinct prion isolates can be attributed to the primary structure of PrP. Cell lines from congenic mice will be established to provide a system to analyse scrapie isolate properties in vitro. The question of whether the PrP allotype and species barriers to scrapie transmission reflect host immune responses will be answered by incubation time studies in SCID (severe combined immunodeficiency) mice. Preferential conversion of PrP allotypes to PrPSc as an explanation for scrapie isolate properties on passage through different mouse strains will be addressed by producing allotype-specific antibodies or by direct protein sequencing. The primary structure of PrP cannot be invoked to account for the distinctive behavior of scrapie isolates in the same inbred mouse strain. Immunoaffinity enrichment for PrPSc and denaturation-renaturation experiments address the involvement of a second component of prions in determining scrapie isolate properties. Somatic mutation of the PrP gene may account for sporadic Creutzfeldt-Jakob disease in humans; targeting mutations through homologous recombination in neural cell lines followed by transplantation to syngeneic mice will examine this possibility. Construction of mice which harbor several insertions of multi-copy transgenes will permit determination of the influence of PrP expression of scrapie incubation period and examination of the possibility for spontaneous scrapie through somatic or germline mutation. An understanding of the mechanisms that lead to diverse symptomology, pathology, susceptibility and disease course within the single family of prion disorders will undoubtedly shed new light on degenerative disease in general.